This invention relates to control systems for liquid chromatographs.
Liquid chromatographs may be considered as including a pump system, a pump control system, a sample injector, a chromatographic column, a monitoring system and a collecting system. The pump system includes one or more pumps for supplying solvents to the chromatographic column under the control of the pump control system. The pump control system controls the rate of flow, the pressure and the composition of mixtures of solvents in the solvent stream applied to the chromatographic column. The chromatographic column includes a sample injector, column packing, connections for receiving solvents to elute components of the sample for sensing by monitoring systems.
In one class of liquid chromatograph, the pressure and flow rate are controlled by circuits which monitor and predict a desired final pressure. In addition to programming a certain rate of flow or pressure of fluid to maximize the separation of constituents of a sample, other provisions for controlling pressure or flow rate are employed to compensate for difficulties with pumps themselves. For example, it has been proposed to reduce the transitory time of a syringe-type pump by attempting to reach a stable pressure for the column ahead of time from previous information.
In one prior art apparatus of this kind, the predicted stable pressure for a column is determined from experience with the particular column. The check-valve of the pump is closed and the pump driven to the known pressure before the valve is opened.
This arrangement has a disadvantage in that it requires knowledge of the column before the pressure of the pump can be programmed.
There are also known arrangements which attempt to compensate for pulsations in a pump by predicting the final pressure of the fluid out of the pump and pressurizing the pump before opening the valve to that pressure to reduce fluctuations. In these arrangements, the pressure is not selected to reduce transitory time but to reduce pulsation from reciprocating pumps. Such systems have the disadvantage of nonetheless having substantial pulsation and not sufficiently reducing transitory time.
Reciprocating piston pumps are popularly used as mobile phase supplies in conventional high performance liquid chromatography. In such systems, flow rates are on the order of 1 to 5 milliliters per minute; pump displacements are on the order of 50 to 100 microliters per stroke; the chromatographic column inside diameters are on the order of 4 millimeters and the volume of the effluent detector at the outlet of the chromatographic column is on the order of 8 to 20 microliters. Sample sizes are on the order of 50 microliters.
Micro-scale analytical high performance liquid chromatographs are known. These chromatographs can attain considerably higher sensitivity by using smaller samples on the order of 1 microliter. Internal column diameters are on the order of 1 or 1/2 millimeter and the effluent detector volume may be on the order of 0.3 microliter in such systems.
The conventional liquid chromatography reciprocating pumps have several disadvantages when used in these micro systems, such as: (1) at the required flow rates, which are well under 1 milliliter per minute, there are deleterious effects of pump check valve leakage, pump seal leakage and compression of the working fluid during the reciprocating cycle resulting in poor flow rate accuracy which makes measurement of retention volumes difficult; and (2) fluctuations in the output pressure and flow rate from these pumps aggravate the already serious problem of noise level in the effluent detector.
Single-stroke, syringe-type pumps do not suffer as much from the flow rate inaccuracy and noise problems of reciprocating pumps, but have the disadvantage of requiring a long transitory time to pressurize the large fluid system after start-up before an equilibrium flow rate is attained. At the lower flow rates used for micro liquid chromatographs, these equilibrium times can be even longer than for normal scale liquid chromatography. This disadvantage of syringe-type pumps is discussed by M. Martin, et al, "The Use of Syringe-Type Pumps in Liquid Chromatography in Order to Achieve A Constant Flow-Rate", JOURNAL OF CHROMATOGRAPHY, 112 (1975) 339-414.
Reciprocating pumps are known in the art, such as from U.S. Pat. Nos. 3,855,129; 3,985,467; 4,131,393 and 4,180,375, which includes systems that measure the pump outlet pressure and modify the action of the reciprocating plunger in the pumps to compensate for the effect of high head pressure on output flow fluctuations and upon output flow accuracy.
These pumps have a disadvantage of not operating with low noise at the low flow rates used in micro liquid chromatography.